It is known in the art of printing machines for securities, for example bank notes, to apply security elements with stamping cylinders. For example, PCT application No WO 96/37368, the content of which is incorporated by reference in the present application, discloses a printing method and apparatus using a hot foil stamping.
In the field of printing securities, it is known to apply a thin layer of metal foil of predetermined shape or pattern to a substrate, for example bank notes, usually as a feature intended to make counterfeiting more difficult. In a known application, the foil is carried as a continuous lamination between a carrier substrate and a layer of hot melt adhesive, the shape or pattern to be applied being determined by a stamp or press. In prior art arrangements the carrier substrate and the substrate are placed together beneath a platen-type press carrying an array of stamps which are simultaneously applied to cause an array of hot foil imprints to be applied to the substrate. Heat for activating the hot melt adhesive is generally applied from the press. The printing apparatus for applying foil to a substrate comprises means for conveying a carrier for the foil together with an adhesive which is activated by heat in a printing position, means for conveying substrate towards said printing position, printing means at which the carrier and substrate are moved together past said printing position at which pressure is applied to transfer a predetermined pattern or shape of foil from the carrier to the substrate, and pre-heating means for transferring heat to the carrier before it reaches the printing position and/or before it contacts the substrate. Preferably the preheating means is effective to transfer heat to the carrier immediately upstream of the printing position. For example, where the printing means comprises a nip between a print cylinder and a counter cylinder the pre-heating means may be located to be effective as close as possible to said nip. In a preferred arrangement the printing means comprises means for applying both pressure and heat at said printing position. For example, the printing means may comprise a print cylinder carrying one or more stamps defining said predetermined pattern and/or shape, the cylinder carrying heating means in the vicinity of said stamp or stamps.
For example, U.S. Pat. No. 6,263,790, the content of which is incorporated by reference in the present application, discloses a machine for security printing on security paper on which security elements carried by a foil are applied to the security prints by a cylinder comprising heated stamps.
Another prior art publication, U.S. Pat. No. 6,062,134, the content of which is incorporated by reference in the present application, discloses a stamping cylinder comprising a plurality of stamps, said stamps being mounted on the cylinder through adjustable means allowing each of the stamps to be adjusted in the axial direction and in the circumferential direction of the stamping cylinder.
A further prior art is disclosed in U.S. Pat. No. 5,429,044, the content of which is incorporated by reference in the present application. In this patent, a machine for security printing on security papers, especially bank notes, is described, said machine comprising a device for feeding with paper, a device for transporting the paper and a unit for applying optically variable images supported by at least one tape which is brought into contact with the paper such that these images are applied at defined locations on the paper corresponding to the printing marks on security papers. The application unit includes at least one applicator cylinder interacting with a pressure cylinder, the paper moving along between these cylinders. The tape is mounted between two reels arranged in a sector of the applicator cylinder and the tape passes along a generatrix of said applicator cylinder, perpendicularly to the path of the paper, under the action of a servomotor acting intermittently at each application step.
Other prior art publications include DE 41 25 996 C1 which relates to a heatable stamping roller with a hollow drum member comprising an internal heating system and a drum mantle with a stamping profile and at least a segment which is interchangeable and located on the drum mantle. The advantage of this design is that the stamping surface can be changed without having to dismantle the heatable roller section.
Finally, publication GB 2 255 317, the content of which is incorporated by reference in the present application, discloses a foil deposition machine which transfers the foils from a carrier onto a surface to be decorated. As indicated in this prior art, foil transfer is conventionally carried out as a flat-bed process, with a die being mounted oh a flat bed and the surface to which the decorative layer is to be applied (i.e. print material) being mounted on the surface of an impression cylinder. The flat bed with the die moves backwards and forwards relative to the impression cylinder, along a path which is tangential relative to the cylinder. A band of foil is supported above the die. As the die approaches the cylinder surface, the foil is pressed by the die (which has been heated to a suitable temperature) against the print material on the cylinder, and the decorative layer is transferred to the print material. The print material is usually paper or card, but may be of plastics or other materials. When the die moves away from the cylinder, the release agent allows the carrier layer to be released from the decorative layer. In order to avoid the inherent slowness of a flat-bed process, it has been proposed to carry out foil blocking by a rotary process. Hitherto, this has required the construction of cylinders on which the foil blocking stamps are engraved. The construction of such cylinders is very expensive, and because the die surface is integral with the body of the cylinder, there are often problems in ensuring correct registration of the die surface with the print material. In GB 2 255 317, there is provided a foil blocking die mounted on a substrate, which substrate is adapted to be secured around the circumference of a cylinder. The use of a die which can be mounted on a cylinder surface for operation, but can be dismounted from the cylinder surface has the particular advantage that the higher operating speeds possible with rotary equipment, as compared to reciprocating equipment, can be obtained.
A problem with the design of GB 2 255 317 is the presence of a non-continuous surface of the plate carrying the stamps forming an opening, which produces vibrations in the machine when the pressure cylinder passes the opening of the plate. Such vibrations then are the cause of improper application of foil or misregistration of printing. A solution to overcome this problem is to mount the cylinders on laterally displaceable axis, for example with excentrical bearings, to relive the pressure when passing such openings but this design is complicated and expensive.